Hydrocarbon drilling and production systems can include the extraction of natural gas from wellbores in subterranean earthen formations. For ease of transport or storage, the natural gas can be liquefied. The liquefaction process includes condensing the natural gas into a liquid by cooling. The liquefied natural gas (LNG) can then be moved and stored more efficiently. Prior to condensing, the natural gas can be treated or processed to remove certain components such as water, dust, helium, mercury, acid gases such as hydrogen sulfide and carbon dioxide, heavy hydrocarbons, and other components.
Natural gas streams may contain methane, ethane, propane, and heavier hydrocarbons together with minor portions of hydrogen sulfide and carbon dioxide. A particular gas composition may include 85% to 95% methane and 3% to 8% ethane with the balance being propane and heavier hydrocarbons. The ethane plus liquid content of such a gas ranges from 2 to 5 GPM (gallons of ethane liquid per thousand standard cubic feet of gas) and is generally considered or identified as a “lean gas.” However, certain natural gas streams include different compositions. Shale gas, for example, may be “richer” than the “lean gas” noted above, with ethane content ranging from 12% to 23%, ethane plus liquid content of 5 to 11 GPM, and heating values from 1,200 to 1,460 Btu/scf. Such an ethane-rich natural gas stream is generally considered or identified as a “wet gas.” It is noted that a “wet gas” may also refer to a gas composition having a relatively high concentration of components heavier than methane.
It is often necessary for the hydrocarbon liquid content in a wet gas or shale gas stream to be removed to meet pipeline gas heating value specifications. In some cases, a hydrocarbon dewpointing unit using refrigeration cooling is used to remove the hydrocarbon liquid content. However, in some cases, the hydrocarbon dewpointing unit may not be sufficient to meet the pipeline gas heating value specifications. For example, with a wet gas or shale gas, the high heating value of the ethane content may exceed the pipeline gas heating value specifications. Accordingly, a natural gas liquid (NGL) recovery unit is needed to remove the hydrocarbon liquids. In some cases, the NGL contents captured by a NGL recovery unit provide economic value. In other cases, a natural gas where the non-methane component is limited can provide an economic value, such as for vehicle fuels.
Many feed gases are provided to the NGL recovery system at relatively high pressure, such as 900 psig or higher, for example. Such an NGL recovery system includes an expander to expand the lean feed gas to a lower pressure, such as 450 psig, for example, for feeding into the fractionation columns. However, a wet or rich shale gas is initially provided at low pressure.